The open-cycle, coal-fired MHD power system, in addition to having a higher efficiency than other fossil-fueled power systems, has the advantage of a self-contained sulfur removal capability. The potassium seed plays a dual role, both increasing the electrical conductivity of the hot combustion gases by thermal ionization and eliminating sulfur dioxide from the gaseous effluent. The spent seed is collected in various down stream components, predominantly as a mixture of water-soluble salts, these being potassium carbonate and potassium sulfate contaminated with fly ash, a typical analysis of the spent seed being estimated as 95 percent potassium sulfate and potassium carbonate and 5 percent fly ash. Because of the high cost of the seed material, currently about 20 cents per pound for potassium carbonate and about 5 cents per pound for potassium sulfate, and the large quantities of seed material required in a MHD power system, a potassium throw-away system for sulfur removal is possible only if there is an adequate supply of potassium carbonate and a sufficient demand for potassium sulfate. Presently, it is not economically feasible to consider a potassium throw-away system, and it is necessary to exploit the desulfurization capability of the MHD seed material for an economical power system, whereby the recovered potassium sulfate must efficiently be converted to potassium carbonate and reused, while sulfur is being collected and treated rather than being disposed to the environment.
The sulfur removing capability of the potassium carbonate seed is critically important when high sulfur coal such as Illinois number 6 coal, the characteristics of which are set forth in Table I below, is used as a fuel in the coal-fired MHD power system. Calculations have showed that it is necessary that at least about 80 to 85% of the potassium sulfate produced in the MHD power system be reconverted to potassium carbonate for economic reasons while restoring the desulfurization capability of the MHD seed to satisfy current EPA emission limits of about 1.2 pounds sulfur dioxide per million BTUs.
TABLE I ______________________________________ Characteristics of Washed Illinois #6 Coal ______________________________________ Proximate Analysis, wt % Ultimate Analysis, wt % ______________________________________ Moisture 12.05 Moisture 12.05 Ash 10.40 Ash 10.40 Volatile Matter 36.05 Sulfur 3.40 Carbon 41.50 Carbon 60.72 Hydrogen 4.22 Nitrogen 1.14 Chlorine 0.02 Oxygen 8.05 ______________________________________ Ash Analysis (ignited basis), wt % ______________________________________ P.sub.2 O.sub.5 0.08 SiO.sub.2 50.83 Fe.sub.2 O.sub.3 21.13 Al.sub.2 O.sub.3 18.33 TiO.sub.2 0.81 CaO 3.20 MgO 0.96 SO.sub.3 2.30 K.sub.2 O 1.93 Na.sub.2 O 0.38 Undetermined 0.05 TOTAL 100.00 ______________________________________ Heating Value: 11,100 Btu/lb Free Swelling Index: 3.0 Hardgrove Grindability: 56
In order for the MHD power system to function adequately, the potassium sulfate conversion to potassium carbonate must be both economical and time effective. Various commercial processes for the conversion of alkali metal sulfate to the corresponding carbonate are available, but none is both economical and time effective. Representative commercially available processes are disclosed in the Guerrieri U.S. Pat. No. 3,301,010 issued Sept. 10, 1968 and the Markant U.S. Pat. No. 3,127,237 issued Mar. 31, 1964 and the Nylander U.S. Pat. No. 3,134,639 issued May 26, 1964. Although these patents illustrate methods for treating alkali metal sulfates, none provides a sufficiently rapid and economical method for treatment of the large quantities of potassium sulfate produced in an MHD power system.
Additional work has been performed by the Pittsburgh Energy Research Center (PERC) relative to a process for treating potassium sulfate. The PERC process involves treatment of potassium sulfate with carbon monoxide and hydrogen gas at a temperature in the range of between about 700.degree. C. and 800.degree. C. to produce potassium sulfide and carbon dioxide and steam which reacts at about 500.degree. C. to form potassium carbonate with the evolution of hydrogen sulfide. Calculated conversion values for the PERC reduction reaction using pure hydrogen and pure carbon monoxide at 750.degree. C. are 2.5% and 2% conversion per pass, respectively. For the regeneration step at 500.degree. C., the calculated conversion for a mixture of 50% carbon dioxide and 50% steam was only 3.9% per pass.